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Study test question about discontiguous networks.

Answered Question
Jul 23rd, 2012
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   Hi.I Just came upon a study test question concerning discontiguous networks.I've attached an Image.I understand the problem when there's , let's say , three routers In series and the middle router doesn't know to which of the edge routers It should route to when they have lans In the same classful network.
This question confused me a little because there's no router In the middle.

Correct Answer by Jan Hrnko about 5 years 4 weeks ago

Ah I see.


I don't really understand why they can't ping eachother when they're  directly connected.


Well they are not directly connected.


10.25.67.32/27 is directly connected only to A

10.25.67.64/27 is directly connected only to B

Do the routers reject the advertised network because  there connected route to the same network has a lower administrative  distance or a lower metric?


Well yes, they are rejected because of lower-higher AD comparison (directly connected AD is 0, EIGRP summary route AD is 5). Router A and also Router B thinks that the whole 10.0.0.0/8 resides on his side.


Note:

EIGRP automatically (because auto-summary is on) creates a discard route 10.0.0.0/8 Null0. Everything that will be destined to other that directly connected network from range 10.0.0.0/8 will be discarded thanks to this route. This is a problem with RIPv2 which is not that smart as EIGRP.


But as as Giuseppe have said regarding EIGRP and split-horizon rule.


If the other router accepts the route 10/8 it cannot advertise the  SAME 10/8 out the same interface for the split horizon rule that says  that a router will not advertise out an interface an IP prefix learned  on that same interface.


As  a result of this one router the router on the right has no return path  for packets sourced from the IP subnet in 10/8 network and IP  communication is broken.


IF  aut-summary is disabled the two routers advertise two different IP  subnets 10.25.67.32/27 and 10.25.67.64/32 and split horizon is not  triggered anymore.

The same happens if the shared link has IP address in 10/8.

---------------------------

My problem Is I can understand the concept If there were a third router  In between the two trying to ping from It's lan.It wouldn't know which  router to choose.

Well the problem with 3 routers is kind of different. Mainly it leads to incorrect routing table on the middle router. Then the middle router can do load-balance and every second packet is sent to the incorrect router.


Best regards,

Jan

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Jan Hrnko Mon, 07/23/2012 - 06:51
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Hi,


try to upload the image, it is not here. Then we can take a look at the question.


Best regards,

Jan

Jan Hrnko Mon, 07/23/2012 - 07:01
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I explained the SAME problem here! :

https://supportforums.cisco.com/message/3688911#3688911


Try to take a look at it.


A discontiguous network is:

A major network that is separated by another major network.


So here you have a major network of 10.0.0.0/8 (Class A) separated by another major network 192.168.0.0/16 (Class C).

Therefore the 10.0.0.0 is a discontiguous network.


Here is an problem with EIGRP with auto-summary on.

EIGRP performs an auto-summarization each time it crosses a border between two different major networks.


Best regards,

Jan

Reprovoid Mon, 07/23/2012 - 07:10
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Thanks for the Info but I'm still a bit lost.

I don't really understand why they can't ping eachother when they're directly connected.Do the routers reject the advertised network because there connected route to the same network has a lower administrative distance or a lower metric?

My problem Is I can understand the concept If there were a third router In between the two trying to ping from It's lan.It wouldn't know which router to choose.

Giuseppe Larosa Mon, 07/23/2012 - 07:31
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Hello Reprovoid,

the split horizon rule applies to EIGRP and it is part of the game.


Both routers would like to advertise the major network 10.0.0.0/8 for the auto-summary at major network boundary,


Let's suppose that router on the right has been faster and it was the first to advertise 10/8 on the shared link.


If the other router accepts the route 10/8 it cannot advertise the SAME 10/8 out the same interface for the split horizon rule that says that a router will not advertise out an interface an IP prefix learned on that same interface.


As a result of this one router the router on the right has no return path for packets sourced from the IP subnet in 10/8 network connected to router on the left and IP communication is broken.


IF auto-summary is disabled the two routers advertise two different IP subnets 10.25.67.32/27 and 10.25.67.64/32 and split horizon is not triggered anymore.

The same happens if the shared link has IP address in 10/8.


Hope to help

Giuseppe

Jan Hrnko Mon, 07/23/2012 - 07:37
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Giuseppe,


you are right! It did not came to my mind . Sorry for my mistake - I will edit the post and add this so that others won't be confused.


Thanks!!!


Best regards,

Jan

Peter Paluch Mon, 07/23/2012 - 08:22
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Giuseppe,


It appears to me that the issues with split horizon here are more convoluted. Please bear with me as I explain and correct me if I am wrong in any part of my reasoning.


The split horizon rule applies, to my best knowledge, only to those networks that are learned by a particular routing protocol, not to networks originated by the router itself. As both routers here are performing automatic summarization, they are both originators of the 10.0.0.0/8 route and they both advertise it as if it was a directly connected network.


Taking your example, if the right router was faster and advertised the summary network 10.0.0.0/8 sooner, this network would indeed be placed into left router's routing table, learned via an IGP. However, this left router also performs automatic summarization and also has the 10.0.0.0/8 network to advertise. Note, however, that even though this network is identical to the network 10.0.0.0/8 in the left router's routing table, it is not the same network - because it is being locally originated and its metric is derived from locally summarized networks - not from the metric of the learned 10.0.0.0/8 route. Therefore, while the left router has the network 10.0.0.0/8 learned in its routing table, it would nevertheless advertise its own summary network 10.0.0.0/8 to the right router.


I've made a quick test in Dynamips to see if this assumption holds. The results were somewhat confusing. RIPv2 behaved exactly as I described above. EIGRP tried to be smarter and actually appears to somehow apply the split horizon rule, corroborating your story.


The bottom line here is - the handling of advertising a self-originated summary route is implementation-dependent and may differ between routing protocols.


Best regards,

Peter

Giuseppe Larosa Mon, 07/23/2012 - 08:31
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Hello Peter,

if both routers would be able to advertise and accept the 10/8 on the shared link there wouldn't be any IP communication failure (and this was my first thought)


I tried to give a possible reason why there may be a communication issue in this setup with only two routers, there may be other possible aspects I haven't considered like the ones mentioned by Jan.

I agree that implementation aspects play a key role here.


Thanks for the quick test. (+5p for your good will)


Best Regards

Giuseppe

Correct Answer
Jan Hrnko Mon, 07/23/2012 - 07:23
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Ah I see.


I don't really understand why they can't ping eachother when they're  directly connected.


Well they are not directly connected.


10.25.67.32/27 is directly connected only to A

10.25.67.64/27 is directly connected only to B

Do the routers reject the advertised network because  there connected route to the same network has a lower administrative  distance or a lower metric?


Well yes, they are rejected because of lower-higher AD comparison (directly connected AD is 0, EIGRP summary route AD is 5). Router A and also Router B thinks that the whole 10.0.0.0/8 resides on his side.


Note:

EIGRP automatically (because auto-summary is on) creates a discard route 10.0.0.0/8 Null0. Everything that will be destined to other that directly connected network from range 10.0.0.0/8 will be discarded thanks to this route. This is a problem with RIPv2 which is not that smart as EIGRP.


But as as Giuseppe have said regarding EIGRP and split-horizon rule.


If the other router accepts the route 10/8 it cannot advertise the  SAME 10/8 out the same interface for the split horizon rule that says  that a router will not advertise out an interface an IP prefix learned  on that same interface.


As  a result of this one router the router on the right has no return path  for packets sourced from the IP subnet in 10/8 network and IP  communication is broken.


IF  aut-summary is disabled the two routers advertise two different IP  subnets 10.25.67.32/27 and 10.25.67.64/32 and split horizon is not  triggered anymore.

The same happens if the shared link has IP address in 10/8.

---------------------------

My problem Is I can understand the concept If there were a third router  In between the two trying to ping from It's lan.It wouldn't know which  router to choose.

Well the problem with 3 routers is kind of different. Mainly it leads to incorrect routing table on the middle router. Then the middle router can do load-balance and every second packet is sent to the incorrect router.


Best regards,

Jan

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